1. Field of the Invention
The present invention relates to the transmitting of feedback information in a wireless communication system. More particularly, the present invention relates to an apparatus and method for effectively transmitting feedback information when a Mobile Station (MS) has two Receive (Rx) antennas and one Transmit (Tx) antenna in a broadband wireless access system.
2. Description of the Related Art
A wireless communication system uses an Adaptive Modulation and Coding (AMC) scheme to achieve data transmission optimized for a user channel condition.
In a fast fading environment, feedback information needs to be periodically transmitted to facilitate an operation of the ACM scheme. In a conventional system having one Transmit (Tx) antenna, a Mobile Station (MS) transmits a post Carrier to Interference and Noise Ratio (CINR) through a feedback channel.
Research has been conducted on a Multiple Input Multiple Output (MIMO) scheme using a plurality of Tx/Receive (Rx) antennas. Use of the MIMO scheme is being considered as one method to increase system capacity. For example, a WiMAX wave2 profile includes a 2×2 MIMO in which there are 2 Tx and 2 Rx antennas.
Similarly to a Single Input Multiple Output (SIMO) scheme, feedback information needs to be transmitted in the MIMO scheme. While the number of Rx antennas of the MS is two, the number of Tx antennas of the MS is limited to one due to implementation complexity, costs, etc.
A closed-loop type transmission scheme is based on an instantaneous value of a wireless channel between a Base Station (BS) and the MS and has a characteristic in that system capacity is great in comparison with an open-loop type transmission scheme. However, the closed-loop scheme requires a large amount of feedback information to obtain correct instantaneous information.
A feedback method includes an analog feedback method and a digital feedback method. In the analog feedback method, the MS transmits a known pilot signal such as a downlink preamble, and the BS directly estimates an instantaneous value of a wireless channel. In the digital feedback method, a downlink channel is estimated, quantized, and transmitted by the MS.
In the WiMAX standard, a sounding method is an example of an analog feedback method. A feedback method using a codebook is an example of a digital feedback method.
In case of using a digital feedback method, the aforementioned problem that the MS has one Tx antenna and two Rx antennas (i.e., 1Tx-2Rx problem) can be addressed by transmitting feedback information for each antenna. However, in this case, the feedback overhead is doubled. Also, in the case of using the analog feedback method such as the sounding method, the aforementioned problem cannot be solved if a Tx chain of the MS is fixed.
FIG. 1 illustrates a 1Tx-2Rx problem of a conventional analog feedback method such as a sounding method.
Referring to FIG. 1, in a WiMAX system using the sounding method for feedback, beamforming and closed-loop MIMO schemes are applied according to a sounding signal transmitted from a BS 110 to an MS 120. In this case, since only one Tx antenna exists, an actual channel condition is not properly estimated. As a result, interference may unexpectedly occur between antennas, which leads to a problem that a large capacity gain cannot be obtained.
Accordingly, there is a need for an apparatus and method for solving the 1Tx-2Rx problem of the analog feedback method such as the sounding method.